The writer is clearly identified as John the Apostle, the son of Zebedee (Matt. 10:2), a prophetic witness and disciple of Jesus, and the writer of the Gospel of John (John 1:1; Rev. 1:1, 3-4, 9; 22:6-10, 18-19). Obviously, he was a Jew, shown by his use of the Old Testament and Targums (the Aramaic Hebrew Scriptures) and his knowledge of the Temple. He knew the Scriptures well. The author was also a significant church leader who spoke with great authority and was well known to the seven churches of Asia Minor, making a pseudo author or a second or third century writing impossible.
Johannine authorship was accepted by the churches in Asia Minor without question as he was well known and it would have been impossible to forge or be misrepresented. The Early Church also identifies John as the author, including Justin Martyr, Ireneaus, Clement of Alexandria, Hippolytus, and Origen.
A few of the later Early Church Fathers, including African bishop Dionysius, concluded that the style of the Gospel of John was scientifically different from Revelation and thus one of them may not have been written by him. But, in later letters, Dionysius also said that it was plausible that John did write both. Liberal scholarship that sometimes seeks to minimize and neuter God’s Word will contend for various other writers and very late writings, using their preconceived personal agendas instead of honest scholarship.
There are some problems in the Gospel of John and Revelation mainly due to some word usages and stylistic differences. However, what is in common as word usage is greater than what is in variance, contending for a singular author for both as well as his three epistles. When one closely examines Revelation, it can easily be seen that the genre is quite different being apocalyptic rather than narrative in nature. Therefore, obviously, there will be different words and styles imposed because the literature style requires it. John also borrows images and vocabulary from the book of Ezekiel as well as Daniel and Zechariah, contributing to the variances. Furthermore, any good writer can write in various genres as many do today, not to mention a possible time lapse between the Gospel (60-90AD) and Revelation (95AD).
But ultimately, the human hands that penned the words or the secretary who dictated them is irrelevant as to the True Author – the Inspiration of the Holy Spirit and Jesus Himself (John 1:1, 10-11; 22:16-20).